The present invention relates to a transcoding system, a video encoding apparatus, a stream processing system and a video decoding apparatus for changing a GOP (Group of Pictures) structure and the bit rate of an encoded bitstream obtained as a result of an encoding process based on MPEG (Moving Picture Experts Group) standards.
In recent years, the broadcasting station for producing and broadcasting television programs has been generally using an MPEG technology for compressing and encoding video data. In particular, the MPEG technology is becoming a de-facto standard for recording video data onto a tape or a random-accessible recording medium and for transmitting video data through a cable or a satellite.
The following is a brief description of typical processing carried out by broadcasting stations up to transmission of a video program produced in the station to each home. First, an encoder employed in a camcorder, (an apparatus integrating a video camera and a VTR into a single body), encodes source video data and records the encoded data onto a magnetic tape of the VTR. At that time, the encoder employed in the camcorder encodes the source video data into an encoded bitstream suitable for a recording format of the magnetic tape of the VTR. Typically, the GOP structure of an MPEG bitstream recorded on the magnetic tape is a structure wherein one GOP is composed of two frames. An example of the GOP structure is a structure comprising a sequence of pictures of the types of I-, B-, I-, B-, I-, B- and so on. The bit rate of the MPEG bitstream recorded on the magnetic tape is 18 Mbps.
Then, a central broadcasting station carries out edit processing to edit the video bitstream recorded on the magnetic tape. For this purpose, the GOP structure of the video bitstream recorded on the magnetic tape is converted into a GOP structure suitable for the edit processing. A GOP structure suitable for edit processing is a structure wherein one GOP is composed of one frame. To be more specific, pictures of a GOP structure suitable for edit processing are all I-pictures. This is because in order to carry out edit processing in frame units, the I-picture having no correlation with other pictures is most suitable. In the actual operation to convert the GOP structure, the video bitstream recorded on the magnetic tape is once decoded back into a base-band video data. Then, the base-band video data is re-encoded so as to comprise all I-pictures. By carrying out the decoding and re-encoding processes in this way, it is possible to generate a bitstream having a GOP structure suitable for edit processing.
Subsequently, in order to transmit an edited video program obtained as a result of the edit processing from the central broadcasting station to a local broadcasting station, it is necessary to change the GOP structure and the bit rate of the bitstream of the edited video program to a GOP structure and a bit rate that are suitable for the transmission. The GOP structure suitable for transmission between broadcasting stations is a GOP structure wherein one GOP is composed of 15 frames. An example of such a GOP structure is a structure comprising a sequence of pictures of the types of I-, B-, B-, P-, B-, B-, P- and so on. As for the bit rate suitable for transmission between broadcasting stations, a high bit rate of at least 50 Mbps is desirable since, in general, a dedicated line having a high transmission capacity such as an optical fiber is installed between broadcasting stations. To put it concretely, the bitstream of a video program completed the edit processing is once decoded back into a base-band video data. Then, the base-band video data is re-encoded to result in a GOP structure and a bit rate suitable for transmission between broadcasting stations as described above.
At the local broadcasting station, the video program received from the central broadcasting station is typically subjected to edit processing to insert commercials peculiar to the district where the local broadcasting station is located. Much like the edit processing carried out at the central broadcasting Station, the bitstream of the video program received from the central broadcasting station is once decoded back into a base-band video data. Then the base-band video data is encoded so as to comprise all I-pictures. As a result, it is possible to generate a bitstream having a GOP structure suitable for edit processing.
Subsequently, in order to transmit the video program completing the edit processing at the local broadcasting station to each home through a cable or a satellite, the GOP structure and the bit rate of the bitstream are converted into respectively a GOP structure and a bit rate that are suitable for the transmission to each home. A GOP structure suitable for the transmission to each home is a structure wherein one GOP is composed of 15 frames. An example of such a GOP structure is a structure comprising a sequence of pictures of the types of I-, B-, B-, P-, B-, B-, P- and so on. A bit rate suitable for transmission to each home has a typical value of as low as about 5 Mbps. The bitstream of a video program often completing the edit processing is decoded back into a base-band video data. Then, the base-band video data is re-encoded into a GOP structure and a bit rate suitable for transmission to each home.
As is obvious from the above description, a video program transmitted from the central broadcasting station to each home is subjected to repetitive decoding and encoding processes for a plurality of times during the transmission. In actuality, various kinds of signal processing other than the signal processing described above are carried out at a broadcasting station and the decoding and encoding processes are usually carried out for each kind of signal processing. As a result, the decoding and encoding processes need to be carried out repeatedly.
However, encoding and decoding processes based on MPEG standard are not 100 percent reverse processed to each other as is generally known. To be more specific, base-band video data subjected to an encoding process is not entirely the same as video data obtained as a result of a decoding process carried out in transcoding of the previous generation. Therefore, decoding and encoding processes cause the picture quality to deteriorate. As a result, there is a problem of deterioration of the picture quality that occurs each time decoding and encoding processes are carried out. In other words, the effects of deterioration of the picture quality are accumulated each time decoding and encoding processes are repeated.